The invention relates generally to voltage multiplier circuits, and relates more specifically to an integrated charge pump circuit with back bias voltage reduction.
Conventional charge pump circuits, such as those shown, for example, in FIG. 1 of this application or in FIG. 1 of U.S. Pat. No. 4,439,692, typically employ a plurality of series-connected diodes having an input terminal, an output terminal and one or more intermediate terminals, with each intermediate terminal being fed by a capacitively-coupled driver. Since the purpose of this circuit is to provide a voltage multiplication, the series-connected diodes in the charge-pump circuit must withstand voltages which exceed the normal power supply voltage range. When a charge pump circuit is required in MOS devices fabricated using standard MOS process technology, it becomes difficult to isolate the relatively high-voltage p-n junctions of these diodes, and additional process steps are usually necessary.
One possible solution to this problem, as shown in U.S. Pat. No. 4,439,692, is to use MOS-configured diodes (MOS transistors connected as diodes) for the conventional diodes of the prior art charge pump circuit. However, because these MOS-configured diodes typically have a larger diode drop (several volts as compared with the 0.7 volt of a conventional-p-n junction), the voltage-multiplying capability of the charge pump is substantially degraded. In other words, to achieve a given output voltage level from the charge pump, the number of cascaded stages in an all-MOS charge pump would be greater than the number of stages in the conventional p-n junction diode circuit. This results in a slower, more complex circuit which occupies additional silicon area. Thus, using prior-art technology, there are substantial drawbacks connected with the otherwise-desirable use of MOS technology in fabricating charge pump circuits.
There are two basic reasons for the relatively large diode drops in MOS-configured diodes. First, in MOS process technology, it is conventional to use a threshold-implant step to force the threshold voltage to between about 1 and 2 volts. Thus, for example, in U.S. Pat. No. 4,439,692, all of the transistors in the charge pump circuit 18 in FIG. 3 are designated as "H" (hard) transistors. In this context, a "hard" transistor is deemed to be one which has a substantially larger positive or negative threshold voltage than that of a so-called "soft" transistor. Thus, as shown in FIG. 4 of U.S. Pat. No. 4,439,692, so-called "hard" transistors may have a threshold voltage of about +1 volt for enhancement mode FET's and a threshold voltage of about -3 volts for depletion-mode FET's. In no case will these "hard" transistors have a relatively low threshold voltage, as the "hard" transistors are by definition those which have a more negative or more positive threshold voltage value.
Secondly, the threshold voltage is further increased by a large body effect caused by large source-to-substrate voltages in integrated circuits employing MOS transistors in the charge pump circuit. This effect occurs because the sources of the MOS transistors cannot be tied to the P-well substrate in which the transistors (typically NMOS devices) are fabricated because the sources must be allowed to rise above the supply voltage to permit the device to function as intended.
In order to create a relatively simple, efficient, fast and compact all-MOS charge pump circuit in an integrated circuit, these problem inherent in the prior-art structures must be overcome.